The production of crude steel is carried out in a blast furnace-converter process or an electric furnace process. Of such methods, the electric furnace process produces steel by melting scrap or reduced iron used as a feed stock through the action of electric energy and, where needed, by further refining the melt. Scrap is now preferred as the feed stock, but reduced iron has of late been in growing demand to compensate for shortages of the scrap, and also to meet the trend toward high quality obtainable by the electric furnace method.
A certain process for the production of reduced iron is known as disclosed in Japanese Unexamined Patent Publication No. 63-108188. This process stacks an iron ore and a solid reducing material in layered formation on a horizontally rotary hearth, followed by reduction of the iron ore with heat applied from above by means of radiant heat transfer, whereby a reduced iron is provided. Such known process is advantageous in respect of possible construction of equipment at rather a low cost, less frequent trouble during operation and the like. In many instances, a horizontally movable hearth takes the form of a rotary hearth as seen in FIG. 1A and FIG. 1B of the accompanying drawings, the two figures being taken to explain the structural details of such rotary hearth. On a movable (rotary) hearth 3 is stacked a layer 1 which is a powdered mixture of an iron ore (oxide powder mainly of a particulate iron ore) and a solid reducing material. The movable hearth 3 is covered by a furnace body 4 lined with a refractory material, and a burner 5 is located upstream in the furnace body. By use of the burner as a source of heat, the iron ore on the movable hearth is subjected to reduction. In FIG. 1A, reference numeral 6 refers to a charging unit for the feed stocks, and 7 to a discharge unit.
In the operation of the foregoing type of furnace, the internal furnace temperature is usually set at around 1300.degree. C. Furthermore, after the reduction is completed, the finished reduced iron is generally cooled on the roraty hearth with the use of a cooling unit and thereafter taken out of the furnace. This prevents such reduced iron from becoming oxidized outside the furnace and also renders the same easy to handle.
The reduction taking place in such method of the production of reduced iron proceeds as a direct reduction between an iron ore and a solid reducing material. The direct reduction is an endothermic reaction, and the supply of heat and the rate of reduction are the determinants of ultimate productivity. The heat supply is conducted to the layer composed of an iron ore and a solid reducing material, by means of radiant heat transfer derived from a burner flame or from an inner wall of the furnace by the use of a burner as a source of heat. With the heat thus supplied, the reduction rate is decided by the thermal conductivity within the layer of iron ore and solid reducing material, and further by the difference in temperature between the reaction temperature and the furnace temperature.
By the term "auxiliary feed stock" used herein is meant a fluxing agent such as limestone, fluorite, serpentine and dolomite.